What Temperature Should Air Be Coming Out Of Vent When Cooling?

In a normally operating air conditioner, the temperature of air coming out of the supply vents should be about 20˚F lower than ambient temperature inside the room. This means that if your room is currently at 90˚F, then the cool air coming out of the supply ducts should be at around 70˚F.

It’s not a fixed value, though. In some cases, the difference can be 25˚F, while sometimes it’s below 15˚F. However, 90% of the time, it’s 18˚F to 20˚F.

This means that if your room is currently at 85˚F, then the temperature of the air coming from the supply vent is likely between 65˚F and 67˚F. This difference is known as Delta T. Delta is the Greek word for “difference” while “T” refers to temperature.

Measuring Delta T in Cooling

Delta T is the temperature difference between the supply vents and the return vents. For simplification;

  • Supply vents: Are the vents that blow air out of the air conditioner into your home/room. The air releases into your room from the AC is known as supply air. During cooling, supply vents carry conditioned, cool air.
  • Return vents: Are the vents through which the AC air handler draws air from your rooms. The air itself is known as return air because it’s returning to the AC for conditioning. Return vents carry stale, unconditioned air.

To measure the difference between the two vents, thus determine the Delta T of your AC, begin by running the AC for at least 15 minutes.

Then, using an infrared thermometer, measure the temperature of the air from the supply register nearest to the evaporator coil for five minutes. Make sure to note down the value. After that, use the same thermometer to measure the return air temperature at the return air grille. Again, wait for five minutes and record the value.

To determine the temperature difference, subtract the two values, i.e., supply air temperature from the return air temperature. You should get a value between 18˚F and 20˚F.

What if the Temperature Difference is Too Low or Too High? Causes and Solutions

Unfortunately, this is a fairly common problem. Sometimes you may find that the air coming out of the supply vents is too warm. Other times it may be too cold.

Temperature Difference Too High

If you find that the difference too high, i.e., above 22˚F, it’s often a sign that the airflow across your coil is too low. This can result from;

Dirty air filter or evaporator

Dirty air filters constrict airflow within the air conditioner. Sometimes it may block airflow altogether. The same problem can arise if the evaporator unit is too dirty. As a result, the air might encounter problems flowing through the system.

Solution: The solution here is straightforward – clean or replace. For filters, you can wash them first. However, if the problem persists, replacement might be the only option. You may need an HVAC professional to clean the evaporator.

A ductwork system that isn’t big enough

This only applies in ducted systems. Nevertheless, it can be a huge problem. If the ductwork isn’t large enough for cool air to flow freely out of the AC and into the house, the supply air might be colder than usual, while the return air may also be warmer than desired.

Solution: This can be a complex problem especially given that ductwork systems are permanent installations. Replacing the entire ductwork can be expensive and time-consuming. It’s best to discuss your options with an HVAC professional.

The fan setting is wrong

Perhaps the fan is set to the wrong speed. Or, maybe the unit is set to “Fan Only” such that it doesn’t cool air. This can cause indoor air to become very hot because fanning doesn’t lower air temperature. Instead, a fan merely creates a breeze that causes a cool sensation across the skin.

Solution: Check your air conditioner to make sure that the unit is set properly. If you will not be around to change the temperatures as ambient conditions change, leave it on the “Auto” setting to self-adjust as the day wears on.

Temperature Difference Too Low

The temperature difference is low if the difference between the supply and return air vent temperatures is below 16˚F. In most cases, it means that the air at the supply end is too hot. This can be caused by

Insufficient refrigerant levels

Refrigerant is the substance that carries the warmth extracted from indoor air and dumps it outside. Now, imagine a situation where there isn’t enough refrigerant in your AC system! It effectively reduces the capacity to remove heat from indoor air. As a result, supply air will still contain substantial amounts of heat.

Solution: The only solution here is to recharge your AC. Keep in mind that you need a license to handle refrigerants in some states. Therefore, you may need to call a licensed technician.

Leaky reverse valves

Reverse valves rarely fail. However, it can cause lots of confusion when it happens. Internal leaks are particularly difficult to troubleshoot as leaking reverse valves and failing compressors have the same symptoms. In the end, though, a leaking reverse valve keeps the compressor pumping gas without corresponding impact on cooling.

Solution: It’s best to involve an HVAC professional here because the reverse valve is a delicate structure soldered to the pipes of copper tubing that go into and out of the compressor towards other parts of the AC. If you attempt DIY, you could cause more damage.

Leaky return air ducts

Finally, the low temperature difference between supply and return valves could also result from leaking return air ducts. Return ducts are on the negative pressure side of the fan.

Therefore, any leak can pull unconditioned air into the home, effectively raising the temperature of the air coming out at the supply end. A few people have done the math and found that even a minor leak in the return air ducts can reduce AC efficiency by up to 70%.

Solution: You also need to involve an HVAC professional here because diagnosing and fixing return air duct leaks is complex and delicate.

Regular Maintenance is Critical 

Proper maintenance is the first step to maintaining a healthy temperature difference between your air conditioner’s supply vents and return vents.

Ensure the filters are clean and wipe the vents periodically to flow in and out of the air conditioner freely. The filters should also be replaced at least once every 1-2 months or are recommended by the manufacturer.

Also, keep the evaporator unit clean and condenser coils cleaner. Dirty coils reduce the efficiency of your air conditioner and could directly affect supply air temperature.

The registers and ductwork must also be clean and free of obstructions. Sometimes you may think that the ductwork is narrow when it’s a pebble blocking it somewhere. Regular inspection and cleaning help prevent such issues.  

Know When to Call an HVAC Pro

Ultimately, you need regular professional support to keep the air conditioner running optimally. Although you only need a professional check once every cooling season, don’t hesitate to call your HVAC technician if the temperature differential issue isn’t going away.